The neutrophil percentage of 78.3% in a three-and-a-half year old child may be caused by physiological factors such as strenuous exercise and emotional excitement, or by infectious diseases such as upper respiratory tract infections and bronchitis. 1. Physiological factors: If you have had strenuous exercise or emotional excitement before the examination, it will lead to a slight increase in the value, which is a physiological phenomenon. 2. Upper respiratory tract infection: Upper respiratory tract infection is caused by pathogens, commonly viruses, bacteria, etc., and the percentage of neutrophils is 78.3% in routine blood tests, and the child will also have symptoms such as coughing, coughing up sputum, and runny nose. 3. Bronchitis: refers to the non-specific inflammation of the trachea, bronchial mucosa and its surrounding tissues, is caused by viral and bacterial infections, the examination will also lead to an increase in the percentage of neutrophils. There are more causes of neutrophil percentage of 78.3% in a three and a half year old child, further examination is recommended to clarify the diagnosis.